Much controversy surrounds the identity and origin of human hepatic stem and progenitor cells in part because of a lack of small animal models in which the developmental potential of isolated candidate cell populations can be functionally evaluated. We show here that adoptive transfer of CD34(+) cells from human fetal liver into sublethally irradiated NOD-SCID Il2rg(-/-) (NSG) mice leads to an efficient development of not only human hematopoietic cells but also human hepatocyte-like cells in the liver of the recipient mice. Using this simple in vivo assay in combination with cell fractionation, we show that CD34(+) fetal liver cells can be separated into three distinct subpopulations: CD34(hi) CD133(hi) , CD34(lo) CD133(lo) , and CD34(hi) CD133(neg) . The CD34(hi) CD133(hi) population contains hematopoietic stem/progenitor cells (HSPCs) as they give rise to T cells, B cells, NK cells, dendritic cells, and monocytes/macrophages in NSG mice and CFU-GEMM cells in vitro. The CD34(lo) CD133(lo) population does not give rise to hematopoietic cells, but reproducibly generates hepatocyte-like cells in NSG mice and in vitro. The CD34(hi) CD133(neg) population only gives rise to CFU-GM and BFU-E in vitro. Furthermore, we show that the CD34(lo) CD133(lo) cells express hematopoietic, hepatic, and mesenchymal markers, including CD34, CD133, CD117, EpCAM, CD73, albumin, a-fetal protein and vimentin and transcriptionally are more closely related to HSPCs than to mature hepatocytes. These results show that CD34(lo) CD133(lo) fetal liver cells possess the hepatic progenitor cell properties and that human hepatic and hematopoietic progenitor cells are distinct, although they may originate from the same precursors in the fetal liver.

Figure Legend:

CD34+ cells from human fetal liver generate hematopoietic and hepatic cells in NSG recipient mice. CD34+cells isolated from cord blood (A) and fetal liver (B) were engrafted separately into sublethally irradiated NSG newborn pups by intracardiac injection. Eight weeks later, PBMC were stained with antibodies for human CD45 (hCD45) and mouse CD45 (mCD45), and analyzed by flow cytometry. Representative hCD45 versus mCD45 staining profiles are shown on the left; the numbers indicate percentages of human and mouse CD45+ cells among total live cells. Paraffin sections of the livers of the same mice were stained with antibody specific for human albumin.

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